It is very often highly desirable to be able to measure electrical power at one or more points in an MMIC. This can be used to control the gain of an active component such as a transmitter or amplifier. It can also be used in diagnostic work or in test evaluation work.
It is often not sufficient merely to know the current passing through a point in the circuit but also the actual power. It is particularly difficult to measure the RF power where the frequency of operation of the circuit is in the range of 1 to 100 Gigaherz (GHz). If the circuit is a resistive circuit the voltage and current will be in phase, but if the circuit is a reactive circuit the power and the current will be out of phase.
Typically it might be required to measure the power over a time interval of 1 millisecond (1 thousandth of a second) and in that time a circuit operating at 50 GHz will have experienced fifty million cycles, and the instantaneous power may vary up and down during those 50 000 cycles.
In apparatus such as power amplifiers oscillators, modulators, mixers, radio data transmission equipment and telecommunications links there has been no practical method of measuring RF power in an MMIC.
In larger circuits it has been possible to tap off a portion of the signal with a coupler and measure the power going through the coupled circuit. However for a coupler to work properly, it has to be a considerable number of wavelengths long, otherwise it is not detecting and coupling with a representative signal. It also uses some of the power of the circuit so is wasteful.
In practise therefore there have been no practical methods of accurately measuring RF power in MMIC's.